1. Technical Field of the Invention
The present invention relates to a digital watermark detection technique. In particular, the present invention relates to a system, method and computer program product for an on-line digital watermark detection, wherein a user terminal authenticates watermark information such as copy protect embedded in images, by using a key generated by a detection center for extracting and encrypting the embedded information and an authentication key generated by the detection center.
2. Description of the Prior Art
As digital media containing images or video images become popular, there arises a need for such a system wherein information identifying an author of a medium is added to the medium and the information is read out as occasion demands.
For example, information concerning permission of duplication is added to a media and a medium playback device is provided with a function of reading and writing this information to prevent illegal duplications.
If in this case the information concerning the permission of duplication is inseparable from the medium itself, the illegal duplications become possible by falsifying the information.
Therefore, the information added to the images must be inseparable from the images.
As one of techniques for solving this problem, there is a digital watermark technique. For example, a digital watermark technique for a still picturexe2x80x9d is a technique for obtaining a watermarked image with some additional information embedded therein by adding an alteration of such a degree as not to be recognized by human vision to an original image transformed to digital data.
By digital watermark embedding, information is embedded in the image itself without greatly hampering the appearance.
There is an example proposed by Ingemar Cox et al. disclosed in JP9-191394 A (1997) and a paper xe2x80x9cSecure spread spectrum watermarking for multimedia,xe2x80x9d IEEE TRANSACTIONS ON IMAGE PROCESSING, vol. 6, No. 12, December, 1997.
In this scheme, an original image is transformed to data (spectra) of the signal domain by using Fourier transform and discrete cosine transform, and noise generated by a pseudo random number is added to a portion considered to be visually important in the spectra in order to embed information.
Furthermore, in Reference 1 by J. R. Smith et al., xe2x80x9cModulation and information hiding in images,xe2x80x9d First workshop on information hiding, Lecture notes in computer science vol. 1174, July 1996, there is shown such a model that digital watermark embedding and detection are grasped as modulation and demodulation in communication.
Many digital watermark schemes currently proposed can be described by this model or a simple modification of this model. Hereafter, the outline of the model will be described.
By the way, in digital watermark schemes, there are schemes using the original image in detection and schemes which do not use the original image in detection.
A system in the present invention is intended for the latter cited schemes. Hereafter, therefore, the system of the present invention will be described as to the digital watermark scheme conducting detection which does not require the original image.
It is now assumed that information V is to be embedded in an image S, in the digital watermark scheme using detection which does not require the original image.
In this case, for data f(S) obtained by transforming S, a watermarked image Sxe2x80x2 satisfying the relation f(Sxe2x80x2)=Wxe2x80x2xc3x97V is generated. As examples of the data f(S), data or a part of data subjected to signal transform such as DCT or Fourier transform, and various statistics such as the moment of the image are considered.
Here, data f(S) is referred to as feature quantity of S. In the case where detection is conducted from the watermarked image Sxe2x80x2, Wxc3x97f(Sxe2x80x2) is derived. However, it is necessary that column vectors of Wxe2x80x2 are orthogonal. Wxe2x80x2 is an inverse matrix of W. In the case where embedded information is V, it is shown by (Equation 1) that the embedded information V is obtained by detection.
Wxc3x97f(Sxe2x80x2)=Wxc3x97Wxe2x80x2xc3x97V=Vxe2x80x83xe2x80x83(Equation 1)
Wxe2x80x2 is a modulation matrix, and W is a demodulation matrix.
For example, in the case where some editing is being conducted for the watermarked image Sxe2x80x2, the (Equation 1) is not valid, but it is considered that an Euclidean distance between the embedded information V and Wxc3x97f(Sxe2x80x2) becomes shorter depending upon the degree of editing.
In this case, V is obtained by quantizing Wxc3x97f(Sxe2x80x2). In the case where one bit of the embedded information is 1 or xe2x88x921, quantization q(X) of a vector X=(x1, . . . , xn) is represented by the following (Equation 2).
q(X)=(xxe2x80x21, xxe2x80x22, . . . , xxe2x80x2m)xe2x80x83xe2x80x83(Equation 2)
xxe2x80x2i=1 if xi is positive
xxe2x80x2i=xe2x88x921 otherwise
For executing detection in the digital watermark scheme coming under this model, it is sufficient to know the transform f( ) and the matrix W.
In other words, f( ) and W can be regarded as a key required to detect the digital watermark. In this case, a person who knows f( ) and W can find which portion of the signal domain of the original image is altered to embed the information.
Therefore, it is possible to falsify the watermarked image. Herein, however, falsifying means to alter a watermarked image so that an originally embedded information will not be detected or information different from the originally embedded information will be detected, without greatly degrading the picture quality of the watermarked image.
In a system using a digital watermark, there is demanded sometimes a condition that every user of a digital watermark can know the embedded information.
Hereafter, two approaches considered to satisfy the above described condition will be described.
First, as an example of a first approach, there is a paper written by Matsui et al., xe2x80x9cAn Application system for Watermark techniques: A protocol for detecting illicit copy images using user""s browsing operationxe2x80x9d, Encryption and information security symposium, 10, 2. C, January 1998.
In a system proposed by this paper, a digital watermark detection program is distributed to users beforehand, and a user singly executes detection by using this program in order to obtain embedded information.
In the system of Matsui et al., key information required for detection is included in the detection program. In the case where a user has known key information required for detection, the user can perform falsification.
Therefore, supposition that users cannot know key information from the detection program becomes necessary. The fact that a program is secure against analyses conducted by malicious users is represented by an expression that the program has tamper resistance.
As the other approach, there is such a system that a reliable center executes the detection and notifies a user of only a detection result. As its example, there can be mentioned a paper written by Yoshiura et al. xe2x80x9cSecure Watermarking System using Public key Cryptographyxe2x80x9d, The Institute of Electronics, Information and Communication Engineers, Conference of Foundation and Boundary, SA-7-7, September 1997. Even if a center which executes the detection answers an illegal detection result to a user, the user cannot gain an insight, in the system of Yoshiura et al. Therefore, to what extent the center executing the detection is reliable becomes important. Furthermore, in the system of Yoshiura et al., it is conceivable that a user executes a part of detection processing such as the transform to the signal domain. In this case as well, it can be said that the possibility of success of falsification is increased by the user analyzing software possessed by the user.
In systems according to the above described two approaches in which anyone of users of a watermarked image can know embedded information, the security against falsification and reliability of the system can be said to be influenced by physical factors such as the tamper resistance of the detection program and the reliability of the center which executes the detection. For solving this problem, it is important to develop such a digital watermark technique that falsification of the embedded information is difficult even if key information required for detection is opened to users. As a concrete system in recent years, there is a paper written by Iwamura et al., xe2x80x9cDigital Watermarks using Error-Correcting Coding for Open Algorithmxe2x80x9d, Cryptograph and information security symposium, January 1999. In the scheme of Iwamura et al., however, there is a problem that a key is needed for each of watermarked images.
In the above described conventional system in which a user can know embedded information of a watermarked image, it is assumed that a center intervenes and conducts detection processing. As a problem in this case, there can be mentioned the fact that the user cannot determine whether an answer of the center is illegal. Furthermore, in the case where a part of detection processing is conducted by a user and the remaining part is conducted by the center, it also poses a problem that software possessed by the user is required to have tamper resistance.
An object of the present invention is to provide a digital watermark detection system in which the user reveives an illegal detection result from a center which executes detection, and it is difficult for a user to falsify an watermarked image even if the tamper resistance is not supposed for a program possessed by the user.
The on-line digital watermark detection system of the present invention is characterized in that the on-line digital watermark detection system includes a user terminal device for encrypting embedded information (watermark) extracted from an input image and authentication information generated beforehand, outputting the embedded information (watermark) and the authentication information thus encrypted, as first transmission information, conducting authentication of second transmission information upon receiving the second transmission information, and extracting and outputting the embedded information decoded from the second transmission information, and a detection center device for decoding the first transmission information upon receiving the first transmission information, and outputting the first transmission information thus decoded, as the second transmission information.
Concretely, the watermark detection system of the present invention may be characterized in that the on-line digital watermark detection system includes a detection center device and a user terminal device, the detection center device includes: demodulation matrix generation means for generating a demodulation matrix required to extract embedded information; extraction and encryption key generation means for transmitting an extraction and encryption key required to extract and encrypt the embedded information to a user terminal device; second transmission information generation means for decoding first transmission information (first information) transmitted by the user terminal device and transmitting the decoded first transmission information to the user terminal device as second transmission information; and authentication key generation means for transmitting an authentication key to the user terminal device, and the user terminal device includes: image input means for inputting an image; feature quantity extraction means for calculating a feature quantity from the image; embedded information extraction and encryption means for extracting embedded information from the feature quantity and encrypting the embedded information by using the extraction and encryption key transmitted by the detection center device; authentication information generation means for generating authentication information required to authenticate the second transmission information; authentication information encryption means for encrypting the authentication information by using the extraction and encryption key; first transmission information generation means for generating the first transmission information and a generation key by using the encrypted authentication information output from the authentication information encryption means and the encrypted embedded information output from the embedded information extraction and encryption means, and transmitting the first transmission information to the detection center device; authentication means for receiving the second transmission information and the authentication key from the detection center device, and authenticating the second transmission information by using the authentication key, the generation key, and the authentication information output from the authentication information generation means; embedded information extraction means for extracting the embedded information from the second transmission information and the generation key; and authentication and embedded information extraction result display means for displaying an authentication result output from the authentication means and the embedded information extraction result output from the embedded information extraction means.
Further, the watermark detection system of the present invention may also characterized in that the on-line digital watermark detection system includes a detection center device and a user terminal device, the detection center device includes: demodulation matrix generation means for generating a demodulation matrix required to extract embedded information; extraction and encryption key generation means for transmitting an extraction and encryption key required to extract and encrypt the embedded information to a user terminal device; second transmission information generation means for decoding first transmission information transmitted by the user terminal device and transmitting the decoded first transmission information to the user terminal device as second transmission information; and authentication key generation means for transmitting an authentication key to the user terminal device, and the user terminal device includes: image input means for inputting an image; feature quantity extraction means for calculating a feature quantity from the image; embedded information extraction and encryption means for extracting embedded information from the feature quantity and encrypting the embedded information by using the extraction and encryption key transmitted by the detection center device; authentication information generation means for generating authentication information required to authenticate the second transmission information; authentication information encryption means for encrypting the authentication information by using the authentication key; first transmission information generation means for generating the first transmission information and a generation key by using the encrypted authentication information output from the authentication information encryption means and the encrypted embedded information output from the embedded information extraction and encryption means, and transmitting the first transmission information to the detection center device; authentication means for receiving the second transmission information from the detection center device, and authenticating the second transmission information by using the generation key and the authentication information output from the authentication information generation means; embedded information extraction means for extracting the embedded information from the second transmission information and the generation key; and authentication and embedded information extraction result display means for displaying an authentication result output from the authentication means and the embedded information extraction result output from the embedded information extraction means.
The on-line digital watermark detection method of the present invention is characterized in that the user terminal section encrypts embedded information extracted from an input image and authentication information generated beforehand, and outputs the embedded information and the authentication information thus encrypted, as first transmission information; the detection center section decodes the first transmission information upon receiving the first transmission information, and outputs the first transmission information thus decoded, as second transmission information; and the user terminal section conducts authentication of second transmission information upon receiving the second transmission information, and extracting and outputting the embedded information decoded from the second transmission information.
The computer program product of the present invention such as, a recording medium is characterized in that the recording medium stores an on-line digital watermark detection program recorded thereon.